Amazon is boosting the price of its monthly Prime membership fees for new and existing members by nearly 20%. The online retailer said Friday its annual membership fee of $99 will not change. From a report: The increase comes less than two years after Amazon first introduced the monthly payment option as a way to attract new Prime members who either couldn't afford the annual membership of $99, which is not increasing, or didn't want to commit to using the service continuously. Prime is the engine at the center of the Amazon commerce machine -- Prime members buy from Amazon more frequently than non-Prime members and also spend more, hence why Amazon introduced the monthly option to lure new members. So if the company is raising the fee, you can bet that it discovered the current $10.99 was just not sustainable.